Microwave radio links in point to point applications require increasing bit rates, some exceeding 1 gigabits per second. While the applicants have disclosed a microwave link with an adaptive rate of modulation is a commonly owned patent application (co-pending U.S. patent application Ser. No. 11/322,972 filed on Dec. 30, 2005 which is incorporated herein by reference), other improvements are desired to meet new application requirements and improve link performance and ease of operation and maintenance. Link performance in terms of rain-fade margins and bandwidth utilization can be increased by using linear modulation such as BPSK and QPSK. At the high bit rates involved, such modulation poses challenges in terms of fast carrier recovery and more flexible rate adaptation with minimum interruption to service. Some applications also require transmitting, in the same link, a variety of traffic, including Ethernet, synchronous Ethernet and legacy high speed data in the form of Sonet/SDH.
Communications service providers who wish to carry legacy PDH services (e.g. T1/E1) over Ethernet sometimes require synchronous Ethernet (SyncE) for carrying the PDH traffic using adaptors. While this technique, known as “pseudo-circuit” is available over conventional Ethernet, some service providers prefer SyncE, in which the Ethernet bit rate clock is synchronized with the PDH bit rate, all of which are synchronized to a network's primary clock. The radio link must accommodate such traffic while providing efficient service to the other types of traffic, including SDH that might operate from a different primary clock. A bandwidth-efficient multiplexing scheme is needed to accommodate these varied streams of traffic, each with different synchronization methods.
The introduction of BPSK/QPSK modulation at high speeds requires implementation of a synchronous modem with quick and efficient carrier recovery system. The high signal bandwidth requires high intermediate frequency (IF), e.g. 4.5 GHz. A noise-robust carrier recovery circuit should support conflicting requirements and have low control-loop bandwidth to maintain high signal to noise ratio (SNR), yet acquire synchronization quickly after brief events of signal fading. When carrier recovery is implemented using a phase locked loop (PLL), the narrow band design cannot lock in at the high slew rate of frequency expected after re-appearance of signal. While this problem exists in many synchronization systems, it is exacerbated when a 4.5 GHz local oscillator attempts to lock on to approximately 450 KHz bandwidth—roughly a 10,000:1 ratio. Thus, special synchronization techniques are needed to handle this limitation of current systems.
These high performance microwave links are subject to deep rain-fading conditions and also to negative link margins during the antenna alignment process. It is desired to devise a low speed digital link between the two radios operating in-band while the deep fade exists. This link may be as slow as 1 kbps, for control and telemetry purpose, and it is called “Local Channel”. It is further desired to accomplish such local channel without a noticeable change in the link's normally radiated spectrum while transmitting the high speed data. This local channel should operate even when the received main signals are so low that radio is unable to synchronize on the incoming high speed carrier phase nor on the modulated symbols clock rate. Finally, this link should be constructed without significant added cost to the existing high-speed link.
Having an improved link with a mixed set of payload streams creates the desire to improve on the link's bit rate adaptation techniques when the link is undergoing a rain fade which is slightly exceeding the link's full speed margin. It is desirable to device the adaptive techniques in a manner that will minimize the down time for higher priority data streams and to increase the number of speed-degradation steps with minimum penalty to system cost or complexity.
Thus, it is desirable to provide a digital microwave system with a variety of ports that addresses the above limitations and it is to this end that the disclosure is directed.